Valve check - bikes vs. cars
From the States:
Valve clearance check: bikes vs. cars. Sorry, pretty lengthy!
Why must it be done more often with bikes? I know the under/over bucket shim system so I will narrow down my answer to address this set-up. Generally, in fact always as far as I know, the clearance decreases over time. If there is no clearance the valves will burn-up because no heat transfer can occur if the valves remain opened which will be the case if the clearance between stem and cam is at zero. The intake valve gets some cooling from the fresh charge, but it is not the case for the exhaust valve. It is the one that usually goes
1- Bike engines (with shims) are designed to rev-up 2 to 3 times higher than car engines. The pounding of the seat is two to three times more frequent. So the valve seats recess a lot quicker into the thin[nest] aluminum heads, thus decreasing the clearance.
2- Bike cam profiles are more radical, thus acceleration and deceleration of the valve are much more brutal. The impact (impulse) into the valve seat is correspondingly more damaging to the valve seat. Guess what? faster recess of the valve seat.
There are other reasons. Car mfgs make more cars of a given model than do bike mfgs. And cars are more regulated with regards to emissions. In fact in the States, everything in a car that has something to do with emissions (and yes this includes the valve train) must carry a ten year warranty – federal mandate. Guess what – lots of thinking is placed into designing the valves for longevity.
Car engines are beefier. I would speculate that there is more meat around the valve seat, slowing down the valve seat recess process, perhaps because of the reason stated above. I drive a Nissan Titan, not sold in the UK. It is a shim/bucket set-up without hydraulically self- adjusted valves. There is no provision for valve check for the life of the engine. In fact the clearances a listed for when the engine is warm, not hot, not cold (cold is std) which does not make any sense. It is a disposable truck (quality is mediocre), so Nissan decided that the valves were good for the life of the truck (V8 revving generally below 2000 rpm).
And finally, perhaps some bike mfgs think that bike riders take care of their rides better than most car owners do their cars. Valve checks are a good place to make money for the dealership.
Read this Honda: I will never, never even consider a bike that does not have a 15,000 miles min. valve check frequency. I had to deal with the 600 miles with my 2001 KLR. Bless Yamaha with their 24,000 miles check. I own a Harley now so no more valve check for me, although I still ride my B-king on occasion. The Busa engine has a good reputation with regards to not requiring adjustments for a very long time, though checks are still required every 15k mi. Bummer. I hope this helps.